This invention relates to an H-switch circuit in use, for example, for driving a DC motor in a forward or reverse direction.
This type of H-switch for controlling the rotation of a DC motor is disclosed in Japanese Patent Disclosure (KOKAI) No. 56-66932, for example. The H-switch is comprised of two pairs of transistors with the emitter-collector paths connected in series between two power sources. Nodes between the emitter-collector paths of the paired transistors are used as output terminals of the switch circuit. The rotating direction of the DC motor is controlled according to the signals applied to the bases of the two pairs of transistors.
In the H-switch circuit, an overcurrent may accidentally flow through the output transistors. If the overcurrent flow continues for more than a given time, the circuit will overheat, leading to the breakdown of the H-switch. Short-circuiting, which is one of the causes of an overcurrent flow in the H-switch circuit, is typically found at some specific circuit locations. In the prior art, H-switch circuit shown in FIG. 1, for example, these locations are:
(1) A load 13 connected between the first and second output terminals 11 and 12. PA1 (2) A path between the first or second output terminals 11 or 12, and a power source V.sub.EE. PA1 (3) A path between the first or second output terminal and a power source V.sub.cc.
In the case of (1) above, the overcurrent flows from Q1 to Q4, or Q3 to Q2. In the case of (2) above, it flows through Q1 or Q3. In the case of (3), it flows through the transistor Q2 or Q4. So far as is known, there has never been an H-switch circuit capable of completely preventing the generation of an overcurrent at various nodes.